Efficient and selective transport of protons is critical in biological contexts (see, e.g., Williams, R. J. P., “Proton circuits in biological energy interconversions”, Annu. Rev. Biophys., Biophys. Chem. 17, 71-97 (1988)) as well as in fuel cell membranes, which are important device components in the quest to move to clean energy sources. K. D. Kreuer, “Proton conductivity: Materials and applications”, Chem. Mater. 8, 610-641 (1996). In biological systems, nature has optimized proton conduction over nanometer-scale dimensions by using secondary and tertiary structures of proteins to precisely arrange appropriate side chains of amino acids, for example in the membrane protein, M2. See, e.g., H. J. Sass, G. Büldt, R. Gessenich, D. Helm, D. Neff; R. Schlesinger, J. Berendzen, and P. Ormos, “Structural alterations for proton translocation in the M state of wild-type bacteriorhodopsin”, Nature 406, 649-653 (2000); J. R. Schnell and J. M. Chou, “Structure and mechanism of the M2 proton channel of influenza A virus”, Nature 451, 591-560 (2008); and A. L. Stouffer, R. Acharya, D. Salom, A. S. Levine, L. Di Costanzo, C. S. Soto, V. Tereshko, V. Nanda, S. Stayrook, and W. F. DeGrado, “Structural basis for the function and inhibition of an influenza virus proton channel”, Nature 451, 596-600 (2008). While controlling proton transfer over nanometer length-scales is adequate for most biological processes, it is essential that efficient proton conduction be obtained over micron length scales for clean energy applications. See, e.g., L. Carrette, K. A. Friedrich, and U. Stimming, “Fuel cells-fundamentals and applications”, Fuel Cells 1, 5-39 (2001); and B. C. H. Steele and A. Heinzel, “Materials for fuel-cell technologies”, Nature 414, 345-352 (2001). For example in hydrogen fuel cells, following oxidation of molecular hydrogen at the anode, the resulting protons must be transported across a selective membrane in order to reach the cathode and complete the conversion of chemical energy to electrical energy. The proton conductivity of this membrane, often called the proton exchange membrane or the polymer electrolyte membrane (PEM), has been one of the bottlenecks to achieving affordable fuel cell technology. Nafion, a poly(tetrafluoroethylene) based polymer with sulfonic acid groups arranged at random intervals along the backbone, is one of the most widely used materials for this membrane. K. A. Mauritz and R. B. Moore, “State of understanding of Nafion”, Chem. Rev. 104, 4535-4585 (2004). The key to proton transport in Nafion is thought to be nanochannels of sulfonic acid groups, through which “hydrated” protons can pass efficiently. See, e.g., O. Diat and G. Gebel, “Proton channels”, Nat. Mater. 7, 13-14 (2008); K. Schmidt-Rohr and Q. Chen, “Parallel cylindrical water nanochannels in Nafion fuel-cell membranes”, Nat. Mater. 7, 75-83 (2008); and J. A. Elliott, S. Hanna, A. M. S. Elliott, and G. E. Cooley, “Interpretation of the small-angle x-ray scattering from swollen and oriented perfluorinated ionomer membranes”, Macromolecules 33, 8708-8713 (2000). Although a good proton conductor for hydrated protons, Nafion suffers from poor conductivity in unassisted proton transfer, i.e., Grotthuss or anhydrous proton transfer, resulting in low conductivities at temperatures above the boiling point of water. M. A. Hickner, H. Ghassemi, Y. S. Kim, B. R. Einsla, and J. E. McGrath, “Alternative polymer systems for proton exchange membranes (PEMs)”, Chem. Rev. 104, 4587-4612 (2004); and M. Rikukawa, and K. Sanui, “Proton-conducting polymer electrolyte membranes based on hydrocarbon polymers”, Prog. Polym. Sci. 25, 1463-1502 (2000). Polymer electrolyte membranes with high proton conductivities at temperatures of 120-200° C. are desirable, since operating at higher temperatures can increase fuel cell efficiency, lower cost, simplify heat management, and provide better tolerance of the catalysts against poisoning. Q. Li, R. He, J. O. Jensen, and N. J. Bjerrum, “Approaches and recent development of polymer electrolyte membranes for fuel cells operating above 100° C.”, Chem. Mater. 15, 4896-4915 (2003). One approach to address this issue is to employ amphoteric functional groups that allow anhydrous proton transport. See, e.g., K. D. Kreuer, “A phenomenon between the solid and the liquid state?”, Solid State Ionics 94, 55-62 (1997); and K. D. Kreuer, A. Fuchs, M. Ise, M. Spaeth, and J. Maier, “Imidazole and pyrazole-based proton conducting polymers and liquids”, Electrochim. Acta. 43, 1281-1288 (1998). Such amphoteric functional groups include imidazole, which is a common motif in biological proton transport in the form of the amino acid histidine. Synthetic polymers containing such amphoteric functional groups have been studied as candidates for high-temperature proton transfer by several groups. See, e.g., G. Scharfenberger, W. H. Meyer, G. Wegner, M. Schuster, K. D. Kreuer, and J. Maier, “Anhydrous polymeric proton conductors based on imidazole functionalized polysiloxane”, Fuel Cells 6, 237-250. (2006); Z. Zhou, S. W. Li, Y. L. Zhang, M. L. Liu, and W. Li, “Promotion of proton conduction in polymer electrolyte membranes by 1H-1,2,3-triazole”, J. Am. Chem. Soc. 127, 10824-10825 (2005); S. Granados-Focil, R. C. Woudenberg, O. Yavuzcetin, M. T. Tuominen, and E. B. Coughlin, “Water-free proton-conducting polysiloxanes: A study on the effect of heterocycle structure”, Macromolecules 40, 8708-8713 (2007); J. C. Persson, P. Jannasch, “Intrinsically proton-conducting benzimidazole units tethered to polysiloxanes”, Macromolecules 38, 3283-3289 (2005); C. B. Shogbon, J.-L. Brousseau, H. Zhang, B. C. Benicewicz, and Y. Akpalu, “Determination of the molecular parameters and studies of the chain conformation of polybenzimidazole in DMAc/LiCl”, Macromolecules 39, 9409-9418 (2006); R. Subbaraman, H. Ghassemi, and T. A. Zawodzinski Jr., “4,5-Dicyano-1H-[1,2,3]-triazole as a proton transport facilitator for polymer electrolyte membrane fuel cells”, J. Am. Chem. Soc. 129, 2238-2239 (2007).
While a number of interesting candidate materials have been identified, there remains a need for materials exhibiting improved anhydrous proton-conduction.